The present invention relates to an electrode which has been specifically designed for carrying out spot welding operations, as well as a method for making it.
As is known, in pressure spot welding operations and, in particular, in resistance spot welding operations, the welding heating is produced by the welding current passing through the contacting surfaces of the pieces to be welded: in this method, the electrode portion which is subjected to the greatest thermal and mechanical stress is the electrode end portion contacting one of the pieces to be welded.
In fact in this portion the welding current, which depends on the electrical resistance of the region to be welded and the voltage applied to the electrodes, is very high so as to reach the welding temperature in a short time in order to prevent heat losses from occurring.
Moreover, a suitable pressure must be applied to the welding region in order to properly clamping the pieces to be welded.
According to known making methods, the welding electrodes which, in addition to a high electric conductivity, must have good resistance characteristics, are made from electrolytic copper, the starting mechanical resistance of which is increased by means of molding or extruding operations.
In order to further increase the mentioned mechanical resistance, there are also used copper alloys (such as elconite, copper-beryllium alloys, copper-zirconium alloys, copper-chromium-zirconium alloys) which however, in respect of electrolytic copper, lead to a greater electric resistance with a consequent decreased conductivity.
Moreover, the shape of the electrode end portions must correspond to the characteristics of the pieces to be welded, and the electrode active surfaces are usually held in a cleaned condition by means of files or emery paper, or, if these surfaces are greatly worn, by means of turning operations.
As it should be apparent the above mentioned maintenance operations are rather tedious and complex and they must be carried out each time the electrode is oxidized and, hence, its active portion has a poor conductivity.
In fact, an oxidized active region leads to a deformation of the electrode operating part, because of the greater current intensity which must be applied to the electrode.
Moreover, as the electrode is very oxidized, the welding voltage is to be increased, in order to provide the required low welding resistance, which leads to the generation of electric arc.
In the switch electric contact field, or in the sliding contact field, there have been already used alloys consisting of silver and colloidal or amorphous graphite, including silver powder with a density of 3g/cm.sup.2, the component elements of said alloys being mixed in a ball mill. Because of the comparatively great size and ductility of the silver powder, said silver powder, during the mixing operation, is laminated and work-hardened and, accordingly, it can not be easily and properly compacted.
According to another method, the silver powder is carried out in a ball mill, by using colloidal water-dispersed graphite: however, also in this method, the silver powder is subjected to lamination and work-hardening.
The mixed silver powder is then compacted, and, during the compaction step, said silver powder is further work-hardened and layered, because of the comparatively large size of the silver powder particles.
The subsequent sintering step is carried out in a very long thermal cycle, of the order of several hours, in which water is slowly evaporated.
The thus obtained sintered material has a hardness from 40.degree. to 75.degree. Brinell, which is much smaller than the hardness for making spot welding electrodes.
In order to improve the mentioned small hardness values, to the silver powder other metal materials are added with a consequent decrease of the electric conductivity.